At cryogenic temperatures, a gas can be stored in a storage vessel in liquefied form to achieve a higher storage density compared to the same gas stored in the gaseous phase. Higher storage density is desirable, for example, when the gas is employed as a fuel for a vehicle because the space available to store fuel on board a vehicle can be limited.
Another advantage of storing a gas in liquefied form is lower manufacturing and operating costs for the vessel. For example, storage vessels can be designed to store a liquefied gas at a cryogenic temperature at a saturation pressure less than 2 MPa (about 300 psig). Compressed gases are commonly stored at pressures above 20 MPa (about 3000 psig), but vessels that are rated for containing gases at such high pressures require a structural strength that can add weight and cost to the vessel. In addition, because of the lower storage density of gas stored in the gaseous phase, the size and/or number of vessels must be larger to hold the same molar quantity of gas and this adds to the weight of the storage vessels if the gas is stored in the gaseous phase. Extra weight adds to operational costs if the vessel is used in a mobile application, either for holding a liquefied gas for transporting it, or for holding the gas on board for use as a fuel to be consumed by the vehicle's engine. For the same molar quantity of gas, the weight of the storage vessels for holding the gas at high pressure in the gaseous phase can be two to five times greater than the weight of the storage vessels for holding the same gas at lower pressure in liquefied form.
The desired temperature for storing a liquefied gas depends upon the particular gas. For example, at atmospheric pressure, natural gas can be stored in liquefied form at a temperature of about 113 degrees Kelvin, and a lighter gas such as hydrogen can be stored at atmospheric pressure in liquefied form at a temperature of about 20 degrees Kelvin. As with any liquid, the boiling temperature for the liquefied gas can be raised by holding the liquefied gas at a higher pressure. The term “cryogenic temperature” is used herein to describe temperatures less than 175 degrees Kelvin, at which a given fluid can be stored in liquefied form at pressures less than 2 MPa (about 300 psig). To hold a liquefied gas at cryogenic temperatures, the storage vessel defines a thermally insulated cryogen space.
A problem with storing a liquefied gas at cryogenic temperatures is providing sufficient thermal insulation to prevent heat transfer into the cryogen space. Conventional vessels use a number of techniques for providing thermal insulation for the cryogen space. For example, double-walled vessels are typically employed with an insulating vacuum provided in the space between the outer and inner walls to reduce convective heat transfer. The outer surfaces of the inner and outer walls can also be wrapped with insulating material to reduce radiant heat transfer. The supports for suspending the inner wall within the outer wall can be designed with an extended heat transfer path to reduce conductive heat transfer.
A pump can be employed to remove the liquefied gas from the cryogen space. When a gas is needed for a high-pressure application, it can be more efficient to use a pump to pressurize the liquefied gas before it is vaporized, compared to using a compressor to pressurize the gas after it has been vaporized. A pump designed for pumping a liquefied gas can be disposed inside or outside the cryogen space.
One of the problems with positioning the pump outside of the cryogen space is that the suction pipe leading from the cryogen space to the pump needs to be well-insulated to prevent the liquefied gas from being heated and vaporized prior to being directed to the pump. Vaporization of any amount of the liquefied gas in the suction line can result in reduced efficiency or inoperability of the pump and/or cavitation, which can damage the pump itself.
When a pump is positioned inside the cryogen space, one of the problems is that heat can be transferred through the pump structure into the cryogen space. In industrial applications, storage vessels are typically stationary installations with a volume that is orders of magnitude greater than the volumes typically used for vehicular fuel tanks. With such large stationary storage vessels, the effect of heat leak through the pump structure is not significant. In smaller mobile storage vessels such as those for carrying fuel on board a vehicle, the same amount of heat leak has a greater effect on the hold time because of the greater proportional effect the heat leak has on a smaller amount of liquefied gas. In large industrial storage vessels it is also possible to lengthen the pump assembly to reduce heat leak by providing a longer heat transfer path. For smaller storage vessels, this approach is limited by the size of the vessel.
Yet another method of reducing heat leak into a storage vessel is to employ a smaller pump that is operated at a higher speed, so that, compared to a larger pump, there is a smaller cross sectional area through the pump and that reduces the amount of conductive heat transfer. A pump operating at a higher speed generally requires more net positive suction head (“NPSH”) to avoid vaporization and cavitation at the pump suction and inside the pump. In stationary industrial installations, because the storage vessel is generally much larger and is not limited by the same size constraints faced by mobile vessels, a stationary storage vessel can be designed with a depth that provides a higher NPSH, allowing the pump to be operated at higher speeds. For example, a stationary storage vessel can be oriented with a vertical longitudinal axis to increase the available NPSH. With a smaller vessel, and especially a mobile one, the vessel's size and orientation can limit the available NPSH, thereby limiting the speed at which a pump can operate.
Given enough time, heat transfer into a cryogen space will eventually cause the vaporization of some of the liquefied gas that is held within the cryogen space, which in turn causes the pressure within the cryogen space to increase. To relieve this pressure, conventional storage vessels typically employ a pressure relief valve to vent some of the vapor from the storage vessel. It is accepted that some heat transfer into the cryogen space will occur with any design. However, it is desirable to reduce the amount of heat transfer to extend the “hold time”, which is defined herein as the length of time a liquefied gas can be held before venting occurs. Longer hold times result in less gas being vented and possibly wasted, more efficient use of energy (since energy is expended to liquefy gases), and for fuel gases like natural gas, this also results in lower emissions fuel into the environment.